Densified silicon nitride has been developed as a structural material for high tempratuare applications and only recently has been discovered to be particularly useful for cutting cast iron, provided the composition is modified by certain second phases (see U.S. Pat. No. 4,323,323). One attempt by the prior art to machine a metal other than cast iron (namely, aluminum) with a spinel modified Si.sub.3 N.sub.4 tool, demonstrated relatively poor results (see Japanese patent No. 49-113803). It has thus become evident that silicon nitride modified to cut one fully dense metal is not universally suitable for cutting various other fully dense metals or materials.
Concurrent with the above development in the prior art, a need has arisen in the ceramic processing arts for efficiently cutting low density metal compacts from which the ceramic is made (see U.S. application Ser. No. 444,901, entitled "Method of Forming Dense Ceramic Bodies", also assigned to assignee hereof). The prior art typically uses tungsten carbide or high speed tool steels to cut low density metal compacts such as silicon. Because of the abrasive nature of silicon particles, these tools have a relatively short cutting life.
This invention deals with the use of a ceramic cutting tool to meet the need in the ceramic processing arts for cutting a low density metal compact to be subsequently converted to a ceramic. The prior art has also attempted to use a dense ceramic (Al.sub.2 O.sub.3) cutting tool to cut a lower density specimen of generally the same ceramic. Such attempt was at relatively low speeds, of limited success, and thus demonstrated the narrow and unpredictable cutting characteristics of ceramics (see "Ceramic Cutting Tools for Machining Unsintered Compacts of Oxide Ceramics" by Holcomb and Rey, Ceramic Bulletin, Vol. 61, No. 12, 1982.